Active Record Inheritance in Rails

Active record Inheritance

ACTIVE RECORD INHERITANCE (ARI)

Oop Basics

1. What is Active Record ?

2. What is Inheritance ?

3. What different Access Specifiers are in Ruby?

4. How they changes method visibilities ?

Types of ARI

1. Single Table Inheritanec (is_a relationship)

2. Multiple Table Inheritanec (has_a relationship)(has_one/has_many)

Implementation step by step

Create new Rails App

rails _2.3.8_ ari -d mysql create create app/controllers create app/helpers create app/models . . . .

1. Single Table Inheritance (is_a relationship)

Generate Product migration

ruby script/generate model product type:string title:string color:string exists app/models/ exists test/unit/ exists test/fixtures/ create app/models/product.rb create test/unit/product_test.rb create test/fixtures/products.yml create db/migrate create db/migrate/20101207145844_create_products.rb

By Sandip Ransing ([email protected]) for JOsh Nights

mailto:[email protected]


Migrate Database

rake db:migrate

(in /home/sandip/ari)== CreateProducts: migrating==========================================-- create_table(:products) -> 0.0018s== CreateProducts: migrated (0.0019s) ===================================

Ruby Console

script/console

Loading development environment (Rails 2.3.8)>> Product.all=> []>> Product=> Product(id: integer, type: string, title: string, color: string, created_at: datetime, updated_at: datetime)

Models

vi app/models/pen.rb cat app/models/pen.rb

class Pen < Product

end

vi app/models/shirt.rb cat app/models/shirt.rb

class Shirt < Product

end



db:migrate


Create initial Data

vi db/seeds.rb cat db/seeds.rb

# This file should contain all the record creation needed to seed the database with its default values.# The data can then be loaded with the rake db:seed (or created alongside the db with db:setup).## Examples:# # cities = City.create([{ :name => 'Chicago' }, { :name => 'Copenhagen' }])# Major.create(:name => 'Daley', :city => cities.first)

Pen.create(:title => 'ball pen', :color => 'red')Pen.create(:title => 'ink pen', :color => 'blue')

Shirt.create(:title => 'Formal', :color => 'White')Shirt.create(:title => 'T-shirt', :color => 'Blue')

Load Data

rake db:seed(in /home/sandip/ari)

Database Record View

script/dbconsole




mysql> select * from products;

+----+-------+----------+-------+---------------------+---------------------+| id | type | title | color | created_at | updated_at |+----+-------+----------+-------+---------------------+---------------------+| 1 | Pen | ball pen | red | 2010-12-07 15:08:46 | 2010-12-07 15:08:46 || 2 | Pen | ink pen | blue | 2010-12-07 15:08:46 | 2010-12-07 15:08:46 || 3 | Shirt | Formal | White | 2010-12-07 15:08:46 | 2010-12-07 15:08:46 || 4 | Shirt | T-shirt | Blue | 2010-12-07 15:08:46 | 2010-12-07 15:08:46 |+----+-------+----------+-------+---------------------+---------------------+4 rows in set (0.00 sec)

Ruby Console

script/console

Loading development environment (Rails 2.3.8)>> Product.all.collect(&:to_s)=> ["#<Pen:0xa5907a4>", "#<Pen:0xa58ff70>", "#<Shirt:0xa58ae6c>", "#<Shirt:0xa58ad2c>"]

>> Pen.all.collect(&:to_s)=> ["#<Pen:0xa57c5d8>", "#<Pen:0xa57c2b8>"]

>> Shirt.all.collect(&:to_s)=> ["#<Shirt:0xa57727c>", "#<Shirt:0xa577100>"]

Rails Log View (Observe mySQL Queries)

Product Load (0.1ms) SELECT * FROM `products` Product Columns (0.9ms) SHOW FIELDS FROM `products`

Pen Columns (0.9ms) SHOW FIELDS FROM `products` Pen Load (0.3ms) SELECT * FROM `products` WHERE ( (`products`.`type` = 'Pen' ) )

Shirt Columns (0.8ms) SHOW FIELDS FROM `products` Shirt Load (0.3ms) SELECT * FROM `products` WHERE ( (`products`.`type` = 'Shirt' ) )

Again On Console (Observe Class Hierarchy)

script/console Loading development environment (Rails 2.3.8)

>> Product.superclass=> ActiveRecord::Base

>> Pen.superclass=> Product(id: integer, type: string, title: string, color: string, created_at: datetime,




updated_at: datetime)

>> Shirt.superclass=> Product(id: integer, type: string, title: string, color: string, created_at: datetime, updated_at: datetime)

>> Product.superclass.superclass=> Object

Observe Public Method

cat app/models/product.rb

class Product < ActiveRecord::Base

validates_uniqueness_of :title

def to_param self.title end

end

On Console


>> p = Product.first=> #<Pen id: 1, type: "Pen", title: "ball pen", color: "red", created_at: "2010-12-07 15:08:46", updated_at: "2010-12-07 15:08:46">

>> p.to_param=> "ball pen"

Observe Private and Protected Methods cat app/models/product.rb


validates_uniqueness_of :title def to_param self.title end protected def buy




end

private # private methods can be called in objects only # can not be invoked with instance # not even self.identity # private methods in ruby accessible to childs you can't completely hide a method(similar to protected in java) def identity puts self.class endend

Ruby Console


>> p = Product.first=> #<Pen id: 1, type: "Pen", title: "ball pen", color: "red", created_at: "2010-12-07 15:08:46", updated_at: "2010-12-07 15:08:46">

>> p.buyNoMethodError: protected method `buy' called for #<Pen:0xb1878c8>

Lets access private method inside class.

cat app/models/pen.rb

class Pen < Product def buy identity "Added to cart" endend

On Console (Observer private methods can be accessed inside class)


>> p = Pen.first=> #<Pen id: 1, type: "Pen", title: "ball pen", color: "red", created_at: "2010-12-07 15:08:46", updated_at: "2010-12-07 15:08:46">

>> p.buyPen




=> "Added to cart"

See the differenec between private and protected

So, from within an object "a1" (an instance of Class A), you can call private methods only for instance of "a1" (self). And you can not call private methods of object "a2" (that also is of class A) - they are private to a2. But you can call protected methods of object "a2" since objects a1 and a2 are both of class A.

gives following example - implementing an operator that compares one internal variable with variable from another class (for purposes of comparing the objects):

def <=>(other) self.identity <=> other.identity end

2. Multiple Table Inheritance (Polymorphic has_a relationship)

Generate migration for Purchase

ruby script/generate model purchase resource_id:integer resource_type:string user_id:integer quantity:integer

Migrate

rake db:migrate(in /home/sandip/ari)== CreatePurchases: migrating======================================-- create_table(:purchases) -> 0.2444s== CreatePurchases: migrated (0.2445s) =================================

Add polymorphic associations in Purchase and Product Model

cat app/models/purchase.rb

class Purchase < ActiveRecord::Base

belongs_to :resource, :polymorphic => trueend

cat app/models/product.rb


has_many :purchases, :as => :resource, :dependent => :destroy validates_uniqueness_of :title

def to_param




self.title end

protected def buy end

private def identity puts self.class endend

On Console


p = Pen.first=> #<Pen id: 1, type: "Pen", title: "ball pen", color: "red", created_at: "2010-12-07 15:08:46", updated_at: "2010-12-07 15:08:46">

>> p.purchases=> []

>> p.purchases.create(:quantity => 2)=> #<Purchase id: 1, resource_id: 1, resource_type: "Product", user_id: nil, quantity: 2, created_at: "2010-12-08 13:47:32", updated_at: "2010-12-08 13:47:32">

>> p.purchases=> [#<Purchase id: 1, resource_id: 1, resource_type: "Product", user_id: nil, quantity: 2, created_at: "2010-12-08 13:47:32", updated_at: "2010-12-08 13:47:32">]




>> Purchase.first.resource=> #<Pen id: 1, type: "Pen", title: "ball pen", color: "red", created_at: "2010-12-07 15:08:46", updated_at: "2010-12-07 15:08:46">

End Of Session !!!

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Active Record Inheritance in Rails

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